1. Technical Field
The present invention relates to wireless communications and, more particularly, wideband wireless communication systems.
2. Related Art
Communication systems are known to support wireless and wire lined communications between wireless and/or wire lined communication devices. Such communication systems range from national and/or international cellular telephone systems to the Internet to point-to-point in-home wireless networks. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards. For instance, wireless communication systems may operate in accordance with one or more standards, including, but not limited to, IEEE 802.11, Bluetooth, Advanced Mobile Phone Services (AMPS), digital AMPS, Global System for Mobile Communications (GSM), Code Division Multiple Access (CDMA), Local Multipoint distribution systems (LMDS), Multipoint Multichannel Distribution Services (MMDS), and/or variations thereof.
Depending on the type of wireless communication system, a wireless communication device, such as a cellular telephone, two-way radio, personal digital assistant (PDA), personal computer (PC), laptop computer, home entertainment equipment, etc., communicates directly or indirectly with other wireless communication devices. For direct communications (also known as point-to-point communications), the participating wireless communication devices tune their receivers and transmitters to the same channel or channels (e.g., one of a plurality of radio frequency (RF) carriers of the wireless communication system) and communicate over that channel(s). For indirect wireless communications, each wireless communication device communicates directly with an associated base station (e.g., for cellular services) and/or an associated access point (e.g., for an in-home or in-building wireless network) via an assigned channel. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated access points communicate with each other directly, via a system controller, via a Public Switched Telephone Network (PSTN), via the Internet, and/or via some other wide area network.
Each wireless communication device includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver (e.g., a station for in-home and/or in-building wireless communication networks, RF modem, etc.). As is known, the transmitter includes a data modulation stage, one or more intermediate frequency stages, and a power amplifier. The data modulation stage converts raw data into baseband signals in accordance with the particular wireless communication standard. The one or more intermediate frequency stages mix the baseband signals with one or more local oscillations to produce RF signals. The power amplifier amplifies the RF signals prior to transmission via an antenna.
As is also known, the receiver is coupled to the antenna and includes a low noise amplifier, one or more intermediate frequency stages, a filtering stage, and a data recovery stage. The low noise amplifier receives an inbound RF signal via the antenna and amplifies it. The one or more intermediate frequency stages mix the amplified RF signal with one or more local oscillations to convert the amplified RF signal into a baseband signal or an intermediate frequency (IF) signal. As used herein, the term “low IF” refers to both baseband and intermediate frequency signals. A filtering stage filters the low IF signals to attenuate unwanted out of band signals to produce a filtered signal. The data recovery stage recovers raw data from the filtered signal in accordance with the particular wireless communication standard. Alternate designs being pursued at this time further include direct conversion radios that produce a direct frequency conversion often in a plurality of mixing steps or stages.
Active mixers used in direct conversion radios, as well as radios that employ an intermediate conversion step, typically comprise input transconductance elements, switches and an output load. These active mixers often have varying output signal characteristics due to environmental conditions, such as temperature, and process and manufacturing variations. These varying output signal characteristics can, for example, result in a mixer producing an errant local oscillation signal that affects the accuracy of an output signal's frequency. Having inaccurate output frequencies can result in many undesirable outcomes, including unwanted signal filtering by a downstream filter. Accordingly, using active mixers that can compensate for the effects of frequency drift that is often present is beneficial. Some current mixers being designed have multiple stages for these and other reasons. Mixers that are designed to have multiple stages, however, utilize significant IC real estate. As the pressure to reduce device sizes continues to drive design, a continuing need exists for multiple mixer stage designs that are more efficient in terms of design and power consumption.
As described above, the trend is to integrate increasing levels of functionality on a single chip. As such, analog circuits, including, for example, analog-to-digital converter circuits, are integrated on the same silicon substrate as digital signal processing circuits to create so-called mix-signal circuits. The integration of analog and digital circuits on one silicon substrate causes substrate noise coupling problems. More specifically, the performance of analog circuits degrades due to substrate noise generated by the digital circuits. For example, clocks utilized on the digital portions of the integrated circuit often produce harmonic tones that propagate through the silicon substrate which cause interference in those analog circuits that perform clock related functions. For example, the harmonic signals from a digital clock may readily interfere with VCO circuits and, more generally, with phase-locked loop circuits.
Substrate noise coupling originates from several sources in addition to harmonic spurs from digital clock related functions. Substrate noise may be generated by the digital circuits propagated through the substrate and impact the analog circuits as well as from capacitive and inductive coupling from any device or block within an integrated circuit. One suspected circuit element that is susceptible to such substrate noise, more specifically, is long interconnect lines that drive high frequency switching clocks. Without a proper solution, such noise can degrade performance dramatically and, in some cases, even destroy a particular functionality. Several techniques have been used to address the problems that arise from substrate noise. Analog circuits are often designed to be adequately robust to withstand the digital noise. Types of techniques that may be utilized include physical separation, using differential architectures, and simulation to develop specific counter measures against detected sources of substrate noise. Another approach is to form deep N-wells. This approach, however, is not always available due to design constraints and has its own costs associated therewith. What is needed, therefore, are integrated circuit designs that reduce substrate noise without creating undue inefficiency and without wasting IC real estate.